Depends on the conditions. If you're underpowered on the 6.0m you'll probably appreciate the 38cm. If you're really over powered a standard 34cm fin might be better, or a 36cm Venom (which is what I use on my Futura 93l).

It's probably best to ride the board with just that fin for the moment. Get a feel for it, and if you're having major control issues when sailing the 6.0m well or overpowered, then consider a smaller fin to increase the board's range. 6.0m -7.0m is not a very big jump and these are ideal sails for the F101 so personally I would just stick to one fin, as new fins are pretty expensive. If your sail gap was bigger than 1m, say 7.5m to 5.5m, then it would probably be worth getting an additional fin.

One side note: shorter fins do not always make it easier to sail overpowered. This is because in heavy chop with a lot of power in the sail it is often easy to get spinout, with the fin always out of the water and a lot of pressure on the back foot. Having a longer fin will save you spinouts. In addition, a bigger fin will trim the board to be higher over the water, making impacts with chop less brutal.